Page 144 - Basic physical chemistry for the atmospheric sciences

130 Basic physical chemistry

ously with all the oxidation half-reactions in Table . 2 that lie above it
6
(since, when these pairs of half-reactions are combined, �ell > 0).
However, since water must be in equilibrium with H + ( aq) , and
O H - (aq), and O H - (aq) are released by the half-reaction (6.29), the
concentration of H + (aq) must decrease. This is achieved through the
half-reaction
2H + (aq) + 2e - � H2(g) (6.30)

which can combine spontaneously with any of the reverse half­
reactions lying above it in Table 6.2 (since combinations of these half­
reactions give �en > 0). Thus , the electrode potential corresponding to
the half-reaction (6. 3 0) defines the lower limit of Eh for natural sys­
tems. Using Eq. (6.26), this lower i mit is
l
0.059 1
Ecell = �ell - -- logQ
n
0 . 0 59 1 [H2(g)]
log 2
= 0 - - 2- [H + (aq)]

= 0 .059 1 log[H + (aq) - ) 0.0295 log[H2(g)]
Since, pH = - log[H + (aq)] , and the partial pressure of hydrogen gas
near the Earth' s surface cannot exceed 1 atm, the minimum value of
Ecell for a system in the presence of water is

Ecell = -0.0591 pH (6.3 1 )

Thus , the redox potential i n natural environments near the Earth' s
surface should not fall below the value given b y Eq. (6.3 1 ) .
The natural limits to p H and E h discussed above are shown i n
Figure 6 . 2 . When the p H and Eh values of other oxidation processes
are plotted on this diagram, it can easily be seen over what ranges
they can be expected to occur in natural environments.

6.9 Gram-equivalent weight and normality
Instead of moles for mass and molarity for concentration, gram­
equivalent weight (or equiv. ) and normality are sometimes used in
considering redox reactions. The equiv. is the amount of a substance
associated with I mole of electrons in a redox half-reaction. For exam­
ple, in the half-reaction (6. 1 6 ) , 1 mole of Zn(s) and I mole of Zn2 + (aq)
are associated with 2 moles of electrons. Therefore, 1 / 2 mole of Zn(s)